In recent years and continuing, orthogonal frequency division multiplexing (OFDM) has been attracting attention. OFDM is robust against multipath propagation and efficient in use of frequency. In an OFDM system, multiple sub-carriers orthogonal to each other are provided in a transmission band, and data values are allocated to the amplitude and the phase of each sub-carrier to perform digital modulation. Because multiple sub-carriers are transmitted in parallel, the frequency band allocated to each sub-carrier becomes narrow. The amount of signals per symbol time is reduced, and therefore, the modulation rate is slow. Meanwhile, because of the low modulation rate, OFDM is insusceptible to multipath interference. In OFDM, an error correction technique such as interleaving and convolution coding is often employed to further reduce degradation in signal reception performance.
FIG. 1 illustrates a typical OFDM receiver 100, in which an RF band processing block 101 performs radio frequency (RF) signal processing. An analog to digital converter (A/D converter) 102 carries out analog to digital conversion. A fast Fourier transform (FFT) block 103 converts time domain signals to frequency domain signals. A pilot signal extraction block 104 extract pilot signals from the received signals. A channel estimation block 105 estimates a channel characteristic of the received data signal based upon the pilot signal. A channel characteristic correction block 106 corrects a channel distortion of the data signal based upon the estimated channel characteristic. A demodulation block 107 demodulates the corrected signal.
As illustrated in FIG. 2, in the channel estimation block 105, a phase and amplitude characteristic calculation block 111 generally calculates variations in phase and amplitude of a pilot carrier to set channel estimation values at pilot positions. Based upon the channel estimation values at the pilot positions, a time direction interpolation block 112 performs interpolation to recover carriers in the time direction. A frequency direction interpolation block 113 interpolates channel estimation values of all the carriers in the frequency direction.
In general, an RF receiver is used in electronic equipment such as cellular phones, personal computers, or peripheral devices. The RF receiver is subject to influence of interfering waves or disturbing waves of high-frequency noise from the electronic equipment. Such interfering waves are narrow-band waves. If the number of sub-carriers (hereinafter, referred to simply as “carriers”) subject to the interference is small, degradation in signal reception performance can be reduced by interleaving and convolution coding.
A technique for making corrections to the position of a FFT window is proposed. See, for example, Japanese Laid-open Patent Publication No. 2000-332727. In this document, the carrier number of an influenced carrier that is under the influence of disturbing waves is estimated. If a predetermined number of valid frequency blocks continue consecutively, then correction is made to the FFT window position based upon impulse response of a reference signal.